Mos device resistant to ionizing radiation

ABSTRACT

An embodiment of a MOS device resistant to ionizing-radiation, has: a surface semiconductor layer with a first type of conductivity; a gate structure formed above the surface semiconductor layer, and constituted by a dielectric gate region and a gate-electrode region overlying the dielectric gate region; and body regions having a second type of conductivity, formed within the surface semiconductor layer, laterally and partially underneath the gate structure. In particular, the dielectric gate region is formed by a central region having a first thickness, and by side regions having a second thickness, smaller than the first thickness; the central region overlying an intercell region of the surface semiconductor layer, set between the body regions.

PRIORITY CLAIM

This application claims priority from Italian patent application No. TO2006A 00785, filed Nov. 2, 2006 which is incorporated herein by reference.

TECHNICAL FIELD

An embodiment of the present invention relates to a MOS device resistant to ionizing radiation, in particular to a power VDMOS (Vertical Double-Diffused Metal Oxide Semiconductor) device, to which the following description will make explicit reference without this implying any loss in generality.

BACKGROUND

As is known, discrete power semiconductor devices used in a space environment, for example, in space systems such as orbital satellites or vehicles for space exploration, typically must intrinsically have a high reliability. In particular, these devices must be resistant to space ionizing radiation, such as electromagnetic radiation of extremely high energy and penetrative power (gamma rays), beams of protons or electrons with energies even much higher than one MeV, cosmic rays constituted by more or less heavy ions having energies even higher than one TeV, or secondary irradiation by electromagnetic beams or beams of particles generated within the same space systems due to interactions with the cosmic radiation. The interaction between the above sources of radiation or particles and the semiconductor device during operation can trigger mechanisms of electrical degradation (ionization) or physical degradation (lattice dislocation), which can evolve in a quasi-instantaneous or cumulative way up to thermal destruction of the device.

In discrete power MOSFET devices, the mechanisms underlying the possible physical degradation or failure are basically of two types, known as:

-   TID (Total Ionizing Dose), i.e., generation of electron-hole pairs     within dielectric layers and consequent accumulation, over time, of     trapped charge, prevalently holes, in active and passive layers of     the device, with progressive degradation of its operating     characteristics; -   SEE (Single-Event Effects), i.e., quasi-instantaneous triggering of     mechanisms of localized energy overload, which can give rise to     marked drifts of the drain local electrical field and to derating of     the drain-source breakdown voltage (process defined as SEB,     Single-Event Burnout), or even to extremely high transient flows of     charge or overvoltage pulses through active dielectric layers, with     consequent permanent dielectric rupture (process defined as SEGR,     Single-Event Gate Rupture).

The above mechanisms are accompanied by an equally harmful degradation (at least in part cumulative) of gate dielectric on the active channel, caused by a drain overcurrent occurring in a transient way due to the interaction between a heavy ion and the active area of the device in the OFF state. In time intervals of a few nanoseconds, an anomalous and intense current of carriers moves in a surface portion of the drain region towards the gate dielectric, and is in part driven by the electrical field towards the channel of the device.

In this regard, reference is made to FIG. 1, showing a VDMOS device 1, comprising in a known manner: a substrate 2 of semiconductor material; an epitaxial surface layer 3, also of semiconductor material, having the same type of conductivity (for example, of an N type) as the substrate 2 and overlying the same substrate 2; a plurality of cells formed in a surface portion of the epitaxial surface layer 3, each comprising a body well 6 having conductivity opposite to that of the epitaxial surface layer 3 (in the example, of a P type), and a source region 7, set within the body well 6, and having the same type of conductivity as the substrate 2; an insulated gate structure 8, constituted by a dielectric gate region 9, formed above the intercell region comprised between adjacent body wells 6 (a surface portion of which is known as neck region, designated by 10) and partially overlapping the body wells 6 and the source regions 7, and by a gate-electrode region 11, formed on the dielectric gate region 9; as is known, the substrate 2 has the function of drain for the VDMOS device 1.

The interaction between a heavy ion and the active area of the device generates a flow of electrons (designated by e) directed towards the substrate 2, and a corresponding flow of holes (designated by p) directed towards the conduction channel of the device (designated by 12 and defined by the portion of the body wells 6 set directly underneath the insulated gate structure 8, and delimited by the junction between the source region 7 and the body well 6 on one side, and by the junction between the body well 6 and the neck region 10, on the other). As highlighted in the detail of FIG. 2 (where field lines are represented with dashed lines), the surface geometry in the active area has a considerable influence in defining the intensity of the electrical-field transverse component E_(t) (i.e., the component directed orthogonally to the surface of the epitaxial surface layer 3). This component, together with the electrical-field longitudinal component E_(l), determines the direction of the resultant electrical field {right arrow over (E)}, which is directed as a whole towards the gate dielectric on the conduction channel 12. A current I directed towards the dielectric gate region 9, or also injected into it, is thus originated, which can entail a progressive charging of the dielectric. This phenomenon can be increased by the effect of current amplification due to the interaction between the charge generated by ionization by the ion and the electrical drain field, and also by triggering of surface conduction of the parasitic bipolar transistor formed by the source region 7, the body region 6, and the drain region (epitaxial surface layer 3).

Typically, technological choices for reducing the incidence of SEGR or charge injection in the dielectric gate region 9 involve the planar geometry of the active area or the thickness of the dielectric gate region 9. In particular, a uniform increase in the thickness of this region (typically constituted of silicon oxide) has proven effective in reducing the probability of SEGR both in the neck region 10 and in the channel region. In fact, the increase in thickness leads to a considerable reduction in the electrical-field transverse component E_(t), and thus in the injection/trapping of charge in the dielectric. At the same time, however, it has been found that thickening of the gate dielectric leads to a deterioration of the resistance to TID mechanism. Frequently, it is difficult to reach a compromise with TID tolerance, in particular in scaled devices, which typically must guarantee a low ON-resistance (Rdson).

SUMMARY

An embodiment of the aim of the present invention is a MOS device that enables a solution to the aforesaid problems and disadvantages, and in particular has a higher resistance to ionizing radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding one or more embodiments of the inventor are now described, purely by way of non-limiting example, with reference to the attached drawings.

FIG. 1 shows a cross section of a MOS device of a known type.

FIG. 2 shows an enlarged detail of the cross section of FIG. 1.

FIG. 3 shows a cross section of a MOS device, according to an embodiment of the present invention.

FIG. 4 shows an enlarged detail of the cross section of FIG. 3.

FIG. 5 shows a cross section through a wafer of semiconductor material in an initial step of a process for manufacturing a MOS device.

FIG. 6 a is a top plan view of a portion of the wafer of FIG. 5 in a subsequent step of the manufacturing process.

FIG. 6 b is a cross section of the wafer of semiconductor material taken along the line VI-VI of FIG. 6 a.

FIGS. 7-11 show cross sections similar to that of FIG. 6 b in subsequent steps of the manufacturing process, according to a first embodiment of the present invention.

FIGS. 12-15 show cross sections through a wafer of semiconductor material in subsequent steps of a process for manufacturing a MOS device, in accordance with a second embodiment of the present invention.

DETAILED DESCRIPTION

As will be detailed hereinafter, an embodiment of the present invention envisages increasing the depth of the MOS device active area via the formation, between the substrate and the epitaxial surface layer, of a further epitaxial layer, having appropriate thickness and resistivity.

In detail, and with reference to FIG. 3 (where parts that are similar to others described previously are designated by the same reference numbers and will not be described again), in a MOS device 20, in particular of a VDMOS type, made according to an embodiment of the present invention, an epitaxial sublayer 21 is set between a substrate 2 and an epitaxial surface layer 3 in which device cells are provided (each comprising, as illustrated previously, a body well 6 and a source region 7). The type of conductivity of the epitaxial sublayer 21 is the same as that of the epitaxial surface layer 3, and its thickness is comparable with that of the epitaxial surface layer 3 (for example, it is comprised between approximately one half and twice the thickness of the epitaxial surface layer). Furthermore, the values of thickness and, mainly, of resistivity of the epitaxial sublayer 21 are chosen in one embodiment so as to maximize the effect of ohmic drop during the ionization transient, when the epitaxial sublayer 21 is traversed by a dense flow of electrons, which are generated by the impact of a heavy ion and by dynamic amplification mechanisms (principally, the aforesaid pre-triggering of the parasitic bipolar transistor) and are forced by the electrical field towards the drain termination.

The presence of the epitaxial sublayer 21 may lead to a double advantage: on the one hand, the active region involved in the drain electrical field E widens downwards, and hence the drain electrical field E decreases at the surface during the ionization transient; on the other hand, the gain of the parasitic bipolar transistor in conditions of strong injection is reduced due to the widening of the effective drain region (collector of the parasitic transistor), with consequent reduction in the current density of surface holes flowing towards the source region 7 and the dielectric gate region 9 during the transient. The presence of the additional epitaxial layer may entail, however, a deterioration in the performance of the MOS device 20, basically as regards the ON-resistance (Rdson); for this reason, the resistivity of the epitaxial sublayer 21 may be between 10% and 50% of the resistivity of the epitaxial surface layer. In one embodiment the resistivity of the epitaxial sublayer 21 does not exceed 50% of the resistivity of the epitaxial surface layer 3.

Another embodiment of the present invention, which is independent of the embodiment described previously but which contributes therewith to increasing the MOS device resistance to ionizing radiation, envisages providing a thicker layer of gate dielectric, exclusively at the neck region 10.

In detail (see again FIG. 3), the dielectric gate region 9 comprises in this case: first portions 9 a, set laterally above the source region 7, the conduction channel 12, and side portions of the neck region 10, and having a first thickness (for example, smaller than 100 nm); and a second portion 9 b, joined to the first portions 9 a, set centrally above just the neck region 10 (in particular, above a central portion thereof) and having a second thickness, greater than the first thickness (by approximately one order of magnitude, for example comprised between 100 nm and 1500 nm). The second portion 9 b has a stripe conformation and extends longitudinally and substantially coaxially with respect to an overlying gate-electrode region 11 (which also has a stripe conformation). Furthermore, lateral surfaces of the second portion 9 b that are connected to the first portions 9 a can be orthogonal to the surface of the epitaxial surface layer 3, or else inclined with respect to the orthogonal direction, for example by an angle of 45°, according to the manufacturing process (as will be clarified hereinafter).

As illustrated also in FIG. 4, the presence of a thicker region of intercell dielectric on the neck region 10 enables a reduction of the transverse component of the electrical field E_(t) (in particular, between gate and drain) on the neck region 10. Furthermore, the same thick intercell region induces indirectly a distortion of the distribution of surface field, which is such as to “smooth” the field lines on the surface and to reduce the transverse electrical field (in particular, between gate and source) on the conduction channel 12.

Furthermore, thanks to the maintenance of a reduced dielectric thickness on the channel region 12, the problem associated to the degradation of tolerance to TID mechanism is strongly limited, at least as regards channel parameters, such as the gate-source threshold voltage (Vgs_(th)).

In greater detail, the lateral and vertical dimensions of the thick intercell dielectric stripe, as compared to the geometrical characteristics of the elementary cells and to the geometry and doping of the surface-drain and body regions, determine how much both the characteristics of breakdown of the MOS device 20 and the resistance to SEE of the same device are modified. In particular, the thickness of the second portion 9 b of the dielectric gate region 9 maybe chosen so as not to lead to an excessive reduction in the breakdown voltage BVdss of the device; for example, this thickness is comprised between 100 nm and 1500 nm and is also a function of the technology used, the voltage class, and the required levels of tolerance to radiation. The width of the second portion 9 b may also be chosen so as to maximize the “planarizing” effect on the surface drain electrical field: the second portion 9 b approaches as much as possible the end of the conduction channel 12, but, for example non-overlapping with the same channel. For example, if the overlying gate-electrode region 11 (which also has a stripe conformation) is 5.7 μm wide and the lateral diffusion of the body wells 6 at the surface is 0.5 μm, the second portion 9 b is, for example, 3.7 μm wide, thus leaving an adequate safety margin from the end of the conduction channel 12, of 0.5 μm on each side.

An embodiment of the manufacturing process of the MOS device 20 envisages first providing a wafer of semiconductor material comprising a substrate 2, and then forming the epitaxial sublayer 21 by epitaxial growth starting from the substrate 2, and forming the epitaxial surface layer 3 by epitaxial growth starting from the epitaxial sublayer 21 (FIG. 5). In particular, the epitaxial growth steps are executed prior to any phototechnique or other surface process. Next, formation of the edge termination, opening of the active area, and cleaning of the surface of the wafer are carried out in a known way and hence not described in detail herein.

Next, the stripe of thick intercell dielectric is formed, according to two alternative variants.

A first variant envisages dielectric formation via CVD. In detail, an optional step of growth of a thin film (not illustrated) of thermal oxide is first carried out before deposition of the thick gate dielectric, and then a CVD deposition of a thick dielectric layer 25 is carried out. The thick dielectric layer 25 can be SG (Silicon Glass) or preferably USG (Undoped Silicon Glass), with thicknesses of from 100 nm to 1500 nm, and the technique of deposition may be PECVD, LPCVD or APCVD; also possible is the use of dielectric multilayers. An (optional) thermal process of dielectric densification is then performed. A second variant envisages, instead, formation of the thick dielectric layer 25 via thermal growth of a dielectric film (silicon oxide, or a compound of oxynitride and silicon oxide) with a thickness of from 100 nm to 500 nm, above the epitaxial surface layer 3. The first variant is hence advantageous when it is necessary to provide large thicknesses of the dielectric stripe.

After formation of the thick dielectric layer 25, an appropriate phototechnique is carried out for definition of the stripe of intercell-gate dielectric, having a longitudinal extension (FIGS. 6 a, 6 b). In detail, a unidirectional dry etch, or an isotropic wet etch, of the thick dielectric layer 25 can be performed, so as to obtain the second portion 9 b of the dielectric gate region 9 on the epitaxial surface layer 3. The dry etch leads to formation of side walls for the stripe that are orthogonal to the surface of the wafer, and maybe advantageous in markedly scaled devices, for containment of lateral dimensions. Wet etching may be used when the spacing between contiguous cells is not excessively small, and the thickness of the stripe is not excessive (between approximately 100 nm and 200 nm). In this case, the final inclination of the side walls of the stripe may be approximately 45°. It is also possible to envisage a sequential wet/dry etch.

A further step of cleaning of the surface of the wafer is then carried out, followed by the formation, by means of thermal oxidation, of a thin dielectric layer 26 (FIG. 7) laterally with respect to the stripe of gate dielectric. The dielectric can be, for example, oxynitride, or a multilayer of oxide, nitride, and oxide, with a thickness of less than 100 nm. A gate-electrode layer 27 is then deposited above the wafer of semiconductor material; the gate-electrode layer 27 is for example formed by doped polysilicon or by a double layer of doped polysilicon and metal silicide having a thickness of between 100 nm and 600 nm.

At this point, according to a first embodiment of the manufacturing process, a CVD deposition of a first dielectric layer, made of USG, PSG, TEOS, or some other dielectric or dielectric multilayer, is carried out. This is followed by a suitable phototechnique for defining the gate electrode 11 (FIG. 8). A unidirectional etch of the first dielectric layer is first performed, with end-point on the gate-electrode layer 27, so as to form a first dielectric region 28, and then a unidirectional etch of the gate-electrode layer 27, for a given time or with end-point on the thin dielectric layer 26, is carried out.

Next (FIG. 9), in a known way that is not described in detail, a first process of ion implantation and diffusion, for formation of the body wells 6, and then (by means of an appropriate masking) a second process of ion implantation and diffusion for formation of the source regions 7 within the body wells 6 are performed. In particular, after the diffusion, the body wells 6 extend underneath the gate structure, but not underneath the second portion 9 b of the dielectric gate region 9.

Then a CVD deposition is performed of a second dielectric layer 29, which is to form spacers. The second dielectric layer 29 can be made of USG, PSG, TEOS, or some other dielectric or dielectric multilayer with low conformability, and its thickness may depend directly on the planar geometry of the wafer.

After a step of dielectric densification, a unidirectional etch is performed of the second dielectric layer 29 to form spacers 30 at the sides of the gate structure, and hence of the first dielectric region 28 and of the gate-electrode region 11 (FIG. 10). The etch removes also part of the thin dielectric layer 26, exposing the underlying source regions 7, and defining the first portions 9 a of the dielectric gate region 9.

There follow optional steps, of a known type, of dry etching for a given time of silicon to form soft trenches, of metal deposition (Co, Pt, or other material) to form a metal silicide, followed by sintering of the film of contact silicide (having a thickness of 10÷100 nm) and by removal of the residual metal from the dielectric surfaces, and of deposition of a metallurgical barrier at the contact (Ti/TiN or the like). A photomasking is then carried out for opening gate contacts on pads, with a known technique.

The process terminates with metallization of the front of the wafer, which leads to formation of a metal layer 32 (FIG. 11), constituted, for example, by an aluminium alloy, and with “patterning” of the same metal layer. There follow deposition and photolithography of the final passivation, according to known techniques, and preparation and metallization of the back of the wafer (drain contact of the device).

A second embodiment of the manufacturing process envisages provision of the lateral insulation of the source contacts (in particular from the gate electrode 11) not via spacers as described previously (so-called self-aligned technique), but via the use of a photolithographic technique.

In detail, starting from the structure described in FIG. 7 (after deposition of the gate-electrode layer 27), through an appropriate masking, a unidirectional etch of the gate-electrode layer 27 is performed, for a given time or with end-point on the underlying thin dielectric layer 26 to form the gate electrode 11 (FIG. 12).

There follow processes of ion implantation and diffusion for formation of the body wells 6 and source regions 7, and CVD deposition of an intermediate dielectric layer 33 on the gate electrode 11 and the surface of the wafer (FIG. 13). The dielectric can once again be USG, PSG, TEOS, or some other dielectric or dielectric multilayer.

After a step of dielectric densification, a photomasking is carried out for opening the contacts, and in particular a dry etch or composite (dry/wet) etch of the intermediate dielectric layer 33. The etch removes also part of the thin dielectric layer 26, exposing the underlying source regions 7 and defining the first portions 9 a of the dielectric gate region 9 (FIG. 14).

There follow the final steps described previously, and in particular metallization of the front for formation of the metal layer 32 (FIG. 15).

From what has been described and illustrated above, advantages of the MOS device and the corresponding manufacturing process according to an embodiment of the invention are evident.

In particular, an embodiment of the MOS device 20 has a high resistance to ionizing radiation and to electrical/physical degradation resulting therefrom in so far as it meets the dual need of: reducing the current density of holes flowing towards the gate and source regions during the transient (by means of the epitaxial sublayer 21); and of reducing the transverse component of the electrical field E_(t) between the gate and the drain on the neck region 10 and between the gate and the source on the conduction channel 12 (by means of the stripe of thick gate dielectric).

Given the particular conformation an embodiment of the dielectric gate region 9, having a greater thickness only at the central stripe portion set above the neck region 10, it is also possible to reach a good compromise as regards resistance to the TID mechanism of the MOS device 20.

Furthermore, the composite structure of the dielectric gate region 9 on the elementary cell leads to further advantages in the performance of the MOS device 20, apart from the reliability effects. Amongst these, the most evident is the reduction in the gate capacitance, which leads to a sensible increase in the switching rate of the MOS device 20.

Finally, it is clear that modifications and variations can be made to what is described and illustrated herein, without thereby departing from the scope of the present invention.

In particular, it is evident that what has been described and illustrated can be applied advantageously to power VDMOS devices having any type of edge-termination structure, and with the stripe in the active area having any dimension. The same teachings may be applied in other MOS devices, such as, for example, signal VDMOSs, IGBTs, and MOSFETs both of the N-channel and of the P-channel type. Furthermore, even though the entire description has been referred to the case of a P-channel VDMOS device, extension to the dual case of an N-channel VDMOS device is immediate.

In addition, it is to be noted that the geometrical parameters of the device (for example, in terms of thickness and resistivity of the epitaxial layers, width of the stripe of gate dielectric, spacing between contiguous stripes, longitudinal pitch, and width/shape of the periodic contact cells) may vary according to the required functions, the design technology, and the characteristics required of the device. For example, the second portion 9 b of the dielectric region 9 may even have a width equal to the width of the neck region 10, also in this case not overlapping with respect to the conduction channel 12.

Finally, it is again emphasized that, even though two embodiments of the invention (presence of the epitaxial sublayer 21 and of the thick dielectric stripe limitedly to the neck region 10) have been illustrated in combination and are jointly particularly advantageous (in so far as they enable a simultaneous solution to two main problems caused by ionizing radiation), they can possibly be implemented independently of one another.

The MOS device of 20 FIG. 3 (or a device having fewer than all of the attributes of the device 20) may be disposed on an integrated circuit (IC) such as a processor or memory, which may be disposed within a system such as a computer system. 

1.-11. (canceled)
 12. A process for manufacturing a MOS device resistant to ionizing-radiation, comprising: forming a surface semiconductor layer having a first type of conductivity; forming a gate structure above said surface semiconductor layer, said step of forming a gate structure comprising forming a dielectric gate region above said surface semiconductor layer and forming a gate-electrode region above said dielectric gate region; and forming body regions having a second type of conductivity within said surface semiconductor layer, laterally and partially underneath with respect to said gate structure; wherein forming said dielectric gate region comprises forming a central region of said dielectric gate region, having a first thickness, above said surface semiconductor layer, and, subsequently, forming side regions of said dielectric gate region, having a second thickness, smaller than said first thickness.
 13. The process according to claim 12, further comprising providing a substrate, and forming a semiconductor sublayer, having said first type of conductivity, on said substrate; wherein said step of forming said surface semiconductor layer comprises forming said surface semiconductor layer on said semiconductor sublayer.
 14. The process according to claim 13, wherein said step of forming a semiconductor sublayer comprises epitaxial growth of said semiconductor sublayer starting from said substrate and said step of forming a surface semiconductor layer comprises epitaxial growth of said surface semiconductor layer starting from said semiconductor sublayer.
 15. The process according to claim 13, wherein said semiconductor sublayer has a resistivity comprised between 10% and 50% of the resistivity of said surface semiconductor layer, and a thickness comprised between 50% and 200% of the thickness of said surface semiconductor layer.
 16. The process according to claim 12, wherein forming said central region comprises depositing a layer of dielectric material above said surface semiconductor layer.
 17. The process according to claim 12, wherein forming said central region comprises thermal growth of a layer of dielectric material on said surface semiconductor layer.
 18. The process according to claim 16, wherein forming said central region further comprises etching said layer of dielectric material by means of a dry etch; or else by means of a wet etch so that said central region has side walls inclined with respect to a direction orthogonal to the surface of said surface semiconductor layer, in particular by an angle of 45°.
 19. The process according to claim 18, wherein said step of etching said layer of dielectric material is such that said central region has a stripe conformation with longitudinal extension.
 20. The process according to claim 12, wherein said step of forming side regions comprises, after said step of forming a central region, forming a dielectric layer above said surface semiconductor layer, joined continuously to said central region; and wherein said central region has a thickness comprised between 100 nm and 1500 nm, and said dielectric layer has a thickness smaller than 100 nm.
 21. The process according to claim 20, wherein said step of forming body regions (6) comprises introducing first dopant species in said surface semiconductor layer so that said body regions extend underneath said dielectric layer, but not underneath said central region; said central region extending consequently above an intercell region of said surface semiconductor layer, set between said body regions.
 22. The process according to claim 21, wherein said step of forming a gate-electrode region comprises forming a layer of conductive material above said central region and said dielectric layer, and subsequently etching said layer of conductive material so that said gate-electrode region extends on said central region, longitudinally and coaxially thereto, and laterally with respect to said central region on said dielectric layer; said step of introducing first dopant species being subsequent to said step of forming a gate-electrode region.
 23. The process according to claim 21, further comprising introducing second dopant species in said body regions, for formation of source regions having said first type of conductivity; wherein said step of forming side regions further comprises, subsequent to said step of introducing second dopant species, the step of etching said dielectric layer so that said side regions extend above channel regions of said MOS device, set between said source regions and said intercell region, and side portions of said intercell region. 24.-43. (canceled)
 44. A method, comprising: forming a gate insulator over a semiconductor layer having a first conductivity, the insulator including a mid portion having a first thickness and including an edge portion having a second thickness that is less than the first thickness; forming a gate electrode over the gate insulator; forming a body region having a second conductivity in the semiconductor layer beneath the edge portion of the gate insulator; forming a source region having the first conductivity in the body region.
 45. The method of claim 45 wherein forming the source region comprises forming the source region beneath the edge portion of the gate insulator.
 46. The method of claim 45, further comprising forming a spacer over the edge portion of the gate insulator and adjacent to a side of the gate electrode.
 47. The method of claim 45, further comprising forming an insulator layer over the edge portion of the gate insulator and over a top and side of the gate electrode. 